In cryogenic distillation plants, a number of distillation columns are contained in a cold box so as to insulate them from the ambient heat. The height of the cold box is generally determined by the length of the longest column. At the present time, in order to produce argon with less than 1 ppm of oxygen cryogenically, up to 250 trays have to be fitted into the argon column which makes the cold box very tall and causes instability problems. In addition, it contains many empty spaces which must be filled with insulating material.
DE-A-2633272 describes a double column with a low-pressure column in two sections, the tops of the two sections being at the same height.